The present invention relates to control systems of the type having a network of distributed programmable controllers connected together through a communications link and, more particularly to a method and means for controlling high speed transmission of relatively small amounts of data among controllers used for transfer line applications or sequential processes which require interlocking among the programmable controllers.
Increasing usage of programmable controllers has resulted in two basic requirements for communications with the controllers, the first being known as SCADA or Supervisory Control And Data Acquisition and the second being known as TCC or Tightly Coupled Control across the controllers wherein data or input/output (I/O) information is transferred from one controller to another. Local area networks exist which are suitable for the SCADA function however, in general, these networks do not provide a cost effective means for the tightly coupled control function.
Programmable Controllers, such as the Series 500 family of PCs, manufactured and sold by Texas Instruments Incorporated, typically programmed in ladder logic, used for machine control, material handling equipment, and batch process control have been interlocked using cross wired word I/O modules. The Series 500 word I/O module of Texas Instruments, for example, can transmit or receive up to eight parallel 16-bit data words per PC scan and if wired with 16 wires can be used for interlocking data for a limited distance. However the need for providing 16 data lines for both input and output along with strobe and ground lines serves as an undesirable limitation on its usefulness from an economic standpoint.
Other prior art approaches in providing peer-to-peer interlocking of data include time division multiple access techniques in which a fixed amount of time is given to each PC to broadcast its data to a supervisory controller. This technique tends to be inefficient and inflexible since it does not conform to varying needs of the network. That is, for example, if certain PCs are coupled to the network but are not active at a given time, that portion of the scan dedicated to the inactive PCs is not utilized. Still other approaches involve collision detection techniques however these approaches have the disadvantage of having an unpredictable time scan. In many cases where peer-to-peer interlocking of data is used, for example where one is synchronizing the operation of machines, it is important to provide a predictable as well as a fast scan time.